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8-VSB

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8-VSB is the 8-level vestigial sideband modulation method adopted for terrestrial broadcast of the ATSC digital television standard in the United States and Canada.

Contents

Throughput

In the 6MHz (megahertz) channel used for broadcast ATSC, 8-VSB carries 19.39Mb (megabits) of usable data per second, although the actual transmitted bit rate is significantly higher due to the addition of forward error correction codes. The eight signal levels are selected with the use of a trellis encoder. There are also the similar modulations 2-VSB, 4-VSB, and 16-VSB. 16-VSB was notably intended to be used for ATSC digital cable, but quadrature amplitude modulation (QAM) has become the industry standard instead.

Power saving advantages

A significant advantage of 8-VSB for broadcasters is that it requires much less energy to cover an area comparable to that of the earlier NTSC system, and it is reportedly better at this than the most common alternative system, COFDM. Some stations can cover the same area while transmitting at an effective radiated power nearly ten times lower. While NTSC and most other analog television systems also use a vestigial sideband technique, the unwanted sideband is filtered much more effectively in ATSC 8-VSB transmissions. 8-VSB uses a Nyquist filter to achieve this, though the filter also causes the transmission to be very "noisy." Receivers must sample the incoming signal at very precise intervals to properly interpret the 8-level signal. Improper timing results in receiving useless information, so this makes reception in moving vehicles nearly impossible due to the Doppler effect. Reed-Solomon error correction is the primary system used to retain data integrity.

In summer of 2005, the ATSC published standards for Enhanced VSB, or E-VSB [1]. This will allow DTV reception on low power handheld receivers with smaller antennas in much the same way DVB-H does in Europe.

Disputes over ATSC's further use

There has been a continuing lobby for changing the modulation for ATSC to COFDM, the way DVB-T is transmitted in Europe, and ISDB-T in Japan. However, the FCC has continued to assert that 8-VSB is the better modulation for use in U.S. digital television broadcasting. In 2000, it denied a petition for rulemaking from Sinclair Broadcast Group requesting the change. Because of continued adoption of the 8-VSB-based ATSC standard in the U.S., a switch to COFDM is now unlikely. Congress passed a law mandating that all analog terrestrial transmissions in the US will be turned off in February 2009, and 8-VSB tuners are increasingly widespread in new TVs due to FCC tuner mandates, effectively mooting this debate.

8-VSB vs DVB-T's COFDM

In 2001, a technical report compiled by the COFDM Technical Group concluded that COFDM did not offer any significant advantages over 8-VSB. The report recommended in conclusion that receivers be linked to outdoor antennas raised to roughly 30 feet (9 m) in height. Neither 8-VSB nor COFDM performed acceptably in most indoor test installations.

However, there were questions whether the COFDM receiver selected for these tests, a transmitter monitor[2] lacking normal front end filtering, colored these results. Retests that were performed using the same CODFM receivers with the addition of a front end band pass filter gave much improved results for the DVB-T receiver, but further testing was not pursued.[3]

The debate over 8-VSB versus COFDM modulation is still ongoing. Proponents of COFDM argue that it resists multipath far better than 8-VSB. Early 8-VSB DTV (digital television) receivers often had difficulty receiving a signal in urban environments. However, newer 8-VSB receivers are far better at dealing with multipath. Moreover, 8-VSB modulation requires less power to transmit a signal the same distance. In less populated areas, 8-VSB often pulls ahead of COFDM because of this. In urban areas, however, COFDM still offers better reception than 8-VSB.

Bifurcation of digital transmission systems

The United States is also notable for creating a separate transmission system for digital radio. An in-band on-channel (IBOC) system developed by iBiquity will be used instead of the Eureka 147 Digital Audio Broadcast system that has been selected in Europe. This is partially due to the fact that the L band normally used for that technology is unavailable in the U.S. However, the American IBOC system uses COFDM, as does Eureka 147 and another standard known as Digital Radio Mondiale.

References

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External links

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